The invention relates to an element for use in an electric circuit having a layer or a layer structure; in which a multi-dimensional charge carrier gas can be formed and in which two regions are situated, which communicate with each other via a channel for the transport of charge carriers.
The term "multi dimensional charge carrier gas" is to be understood to means a three- or two-dimensional gas. A charge carrier gas of the last-mentioned kind is situated, for example, in the proximity of a hetero-junction between a doped semiconductor layer having a relatively large energy gap and an intrinsic semiconductor layer having a relatively small energy gap, the conduction band edge in the last-mentioned semiconductor material behaving energetically at a lower level than in the first-mentioned material. Due to the fact that an energy minimum occurs in the semiconductor material having a relatively small energy gap at a small distance from the junction, a cloud of electrons of very small thickness, i.e. practically a two-dimensional cloud, is generated by transport of electrons from the material having the relatively large energy gap near the hetero-junction. The electrons present in this cloud have a strongly increased mobility due to their spatial separation from the donor ions from which they orginate.
An element of the kink mentioned above is in the form of a HEMT (High Electron Mobility Transistor) known from Nishizawa, "Semiconductor Technologies", Tokyo, 1982, p. 258-271. The HEMT is essentially a field effect transistor, in which the conduction takes place in a practically two-dimensional electron gas. Due to the increased mobility in the electron gas, a high switching speed can be attained with the HEMT. The transistor is normally operated at such a temperature that the electron gas is not completely degenerated. The dimensions of the HEMT are usual for other more conventional types of transistors.
In practically all the elements for use in electric circuits, briefly designated hereinafter as electric elements, deviations in the electrical properties of the element occur. These deviations may be due to spreading in their manufacturing steps. As a result, the electrical properties of the finished products practically always deviate more or less from the values aimed at. Moreover, many electric elements are subjected during operation to ageing processes, as a result of which their electrical properties change.
Besides, deviations may be caused by changes in the environment in which the element is used, such as, for example, variations in the ambient temperature.
Thus, for example, the temperature coefficient of the resistivity of most materials has a value different from zero, as a result of which many elements are more or less sensitive to temperature.